The present invention relates to safety devices for load elevators and, more particularly, automatically actuatable fail safe brake mechanisms for platform lifts of the kind having hollow guide rails or columns along which the platform is raised and lowered.
Trucks are often fitted with powered load lifting and lowering appliances, commonly called lift gates, comprising a platform that is movable between ground level and the bed of the vehicle, either at the side or at the tail end of the vehicle bed. In one type, the apparatus includes a laterally spaced apart pair of vertically extending fixed guide columns each of which mounts a vertically elongate runner assembly, the opposite sides of the load platform being connected to the lower ends of the pair of runner assemblies. Typically, the upper end of each runner assembly is interconnected, within the corresponding guide column, to one end of a flexible wire rope, chain or cable that is trained around a sheave at the upper end of the guide column so that the platform is, in effect, suspended from the cable or the like. The other end of the flexible suspension element is usually interconnected to a unidirectionally acting hydraulic cylinder. Typically, the hydraulic system for the cylinder is such that when it is desired to raise the platform from ground level to bed height, a pump is energized to pressurize that side of the cylinder piston which will effect raising of the platform. Conversely, when it is desired to lower the platform from bed height the pump remains de-energized while a valve control is actuated to bleed the cylinder as the force of gravity lowers the platform.
In a lift of this kind, as the platform is suspended by means of a cable or the like, the platform and its load, if any, may drop suddenly if one of the cables supporting the platform breaks. In order to prevent the platform from then falling to the ground, a safety device has previously been devised of the kind shown in United Kingdom patent specification No. 1,429,181. Briefly, in this device at least one of the runners has a pawl mounted on it to which the cable is connected either directly or through an intermediate mechanism such as a spring biased lever. The pawl is normally held out of frictional engagement with its associated guide member by tension in the cable but pivots into frictional engagement with the inside of the guide member when the tension in the cable is released. However, in this device there is an unsatisfactory delay in the response of the pawl to the loss of tension in the supporting cable. Consequently, after a cable breaks the platform and its load, if any, can drop vertically an unacceptable distance along the guide column. Further, as the response time is relatively long the acceleration due to gravity may become quite large and so increase the difficulty of arresting the momentum of the falling platform by means of the interengagement of the pawl with the guide column.
In addition, with the prior device, after the pawl has been actuated into locking engagement with the inside of the guide column, the pawl cannot be unlocked except by raising the platform and the body of the locking device. However, lifts of this kind are fitted with a stop member at or adjacent the upper end of the guide column against which the upper end of the corresponding runner assembly abuts in the fully raised position of the platform. Thus, if the pawl is locked to the column at a time when the platform is in or adjacent to the fully raised position the device cannot be reset except by at least partially dismantling parts of the apparatus. Accordingly, the prior device requires a second auxiliary stop device in the upper part of the column beneath the platform stop which must be specially adjusted relative to the pawl within the column.
Further, taking into account the limited clearance available within a guide column for mounting the parts of the safety device and the range of sizes of cable end fittings to be connected to the safety device, the prior device is not practically adaptable to use with heavy-duty larger capacity lifts.